No. If I use -s 1452, there will be no fragment and the total length is 1500.
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deepskyOct 5 '11 at 7:22

1

I dont know IP6, but with IP4 I have to set my TCP param MTUs to 1480 because my switcher, it is like a 15' truck wont fit under the bridge */clunk* but a 14' 11" one flies right through. And it still goes 99mph that way. top 1% in speed tests for my connection. so i dont worry about the minor difference
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PsycogeekOct 5 '11 at 8:02

You would see this if you have VLAN tagging on an interface that doesn't support it natively. 802.1Q VLAN tags take an additional 4 bytes. If your NIC supports them natively, it supports 1522-byte (instead of older 1518-byte) Ethernet frames so you can still have a 1500 byte MTU. If your NIC does not support it natively, you can support it in software but you have to shrink the MTU to 1496 so the whole thing fits in the old 1518-byte frames your NIC supports.

There is no VLAN configured on my network. And the Ethernet frame contains no 802.1q information. It's 14Bytes observed from the wireshark.
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deepskyOct 5 '11 at 7:23

4

Actually, I think it's because any new segments created must have an 8-byte boundary, and still be less than the MTU. 1500 is 187 div 8 with a remainder of 4. Thus, 1500-4=1496 would have to be the new boundary. There is greater detail about fragmentation at erg.abdn.ac.uk/~gorry/eg3567/inet-pages/ip-fragmentatiion.html.
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PaulOct 5 '11 at 7:43

I think this makes sense. Not enough reputation to up vote....
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deepskyOct 5 '11 at 8:41

The maximum transmission unit (MTU) of a communications protocol of a layer is the size in bytes of the largest protocol data unit that the layer can pass onwards.
For the example of IP over DSL connections using PPPoA/VC-MUX, again choosing to fill 31 ATM cells as before, we obtain a desired optimal reduced MTU figure of 1478 = 31*48-10 taking into account an overhead of 10 bytes consisting of a Point-to-Point Protocol overhead of 2 bytes, and an AAL5 overhead of 8 bytes. This gives a total cost of 31*53=1643 bytes transmitted via ATM from a 1478 byte packet passed to PPPoA. In the case of IP sent over ADSL using PPPoA the figure of 1478 would be the total length of the IP packet including IP headers. So in this example, keeping to a self-imposed reduced MTU of 1478 as opposed to sending IP packets of total length 1500 saves 53 bytes per packet at the ATM layer at a cost of a 22 byte reduction of the length of IP packets.

RFC 2516 prescribes a maximum MTU for PPPoE/DSL connections of 1492 bytes: the 1500 byte maximum ethernet payload minus 8 bytes of PPPoE headers (2 bytes for the PPP overhead, and 6 bytes for the PPPoE header). This will not necessarily fill an integer number of ATM cells.
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model. You do measure in layer 3, so that the difference is obvious. Because of the kind of network you are making use of 4 bytes are used for network management purposes.